1. Field of the Invention
This invention relates to control devices for air conditioning refrigeration systems and particularly to a device that monitors the super-heat contained in a refrigerant to control operation of the compressor if the super-heat contained in the refrigerant either exceeds or falls below predetermined limits.
2. Description of Prior Art
It is believed that the prior art related to this invention may be found in Class 62, sub-classes 149, 158, 208, 209, 227 and 228. A search through the class and sub-classes indicated has revealed the existence of U.S. Pat. Nos. 3,913,347; 3,047,696; 3,400,552; 3,729,949; 3,303,663; 3,803,864; 3,786,650; 3,130,558; and 3,791,165.
In refrigeration systems there is a so called "suction line" which runs from the evaporator to the compressor. This line normally returns the heat-laden refrigerant in gaseous form from the evaporator to the compressor. The line is so arranged that the refrigerant gas is warmed a few degrees as it picks up heat through the walls of the tubing. Heat may be applied to the tubing in various ways, such as by running the suction line through a heat exchanger so as to draw heat from the high pressure and relatively "hot" liquid refrigerant prior to its presentation to the expansion valve in the system. This method achieves the double function of adding "super-heat" to the refrigerant gas returning through the suction line to the compressor, and "sub cooling" the high pressure relatively "hot" liquid refrigerant prior to passage through the expansion valve. Super-heat may thus be defined as the heat contained in a refrigerant gas beyond the amount required to maintain its boiling point. Since super-heat causes a rise in temperature of the refrigerant gas in its return to the compressor, it is sensible heat. The fact that super-heat can be sensed or detected by the "sensing element" of an instrument is relied upon in U.S. Pat. No. 3,047,696 in which it is recognized that the thermostatic expansion valve which releases high pressure liquid refrigerant in a controlled manner into the relatively low pressure space provided by the evaporator normally controls the super-heat of the refrigerant leaving the evaporator. The super-heat control device disclosed by this patent is related to the control of the air conditioning system of an automobile, and teaches that with the particular refrigerant disclosed by this patent the normal level of super-heat in the suction line is approximately 23.degree. F. The patent discloses that when the super-heat exceeds about 60.degree. F., this is an indication that the refrigerant charge has been lost in the system. Accordingly, under normal conditions, such a loss of refrigerant can cause extensive damage to the compressor if the compressor is not shut down. According to the invention disclosed by this patent, when the exceedingly high super-heat is detected, an electrical circuit is closed which has the effect of blowing a fuse which results in deactivating the compressor unit. To close the electrical circuit, this patent discloses a device that utilizes differential pressure between suction line refrigerant and a second refrigerant which is responsive to the increase in temperature of the suction line refrigerant gas to shift the position of a diaphram carrying an electrical contact.
U.S. Pat. No. 3,130,558 recognizes the destructive effect of a slug of liquid refrigerant admitted to the input port of the compressor. Since most liquids, including liquid refrigerants, are not compressible, and since a compressor is intended to be a vapor pump dependent for its operation upon the elasticity of the vapor it is compressing, the admission of an incompressible slug of liquid refrigerant to the input port of the compressor will obviously result in damage to the compressor. This patent teaches a system for protecting the compressor from such a slug of liquid refrigerant which involves sensing the temperature of the refrigerant in the suction line and applying this temperature to the expansion valve in such a way that liquid refrigerant is normally admitted to the evaporator under controlled conditions that insure that the temperature of the refrigerant leaving the evaporator contains the requisite amount of super-heat.
This interrelationship of temperature of the refrigerant gas as it leaves the evaporator, and control of the expansion valve in relation thereto, is almost universally used in air conditioning refrigeration systems. This patent goes one step further and includes in the suction line a control device including a diaphram enclosed within a housing. Movement of the diaphram in one direction effects closing of electrical contacts which activate a solenoid valve arranged in a bypass line to permit the passage of high pressure and relatively "hot" refrigerant gas to be admitted to the suction line, thereby adding "super-heat" to the refrigerant gas returning to the compressor and eliminating the possibility of a slug of liquid refrigerant damaging the compressor. Movement of the switch-controlling diaphram in one direction is influenced by the pressure within the suction line, as balanced by an appropriate spring, and movement in the opposite direction is influenced by the expansion of an appropriate second refrigerant in the space above the diaphram, expansion of the second refrigerant being controlled by the temperature of the refrigerant gas returning through the suction line to the compressor.
U.S. Pat. Nos. 3,303,663 and 3,400,552 both relate to apparatuses for controlling the charging of a refrigerant into an operating refrigeration system. Both utilize the pressure and temperature characteristics of the returning suction line refrigerant gas for control purposes.
U.S. Pat. No. 3,686,892 teaches the concept of utilizing the temperature of the refrigerant gas returning to the compressor to actuate a switch which in turn energizes a wire heater which in turn opens a thermally responsive fuse to de-energize the compressor circuit.
U.S. Pat. No. 3,729,949 relates to the use of a plurality of movable switch elements that are responsive to temperature and pressure to control the charging of a refrigeration system with an additional charge of refrigerant.
U.S. Pat. No. 3,786,650 relates to an air conditioning control system in which the expansion valve is controlled in such a manner as to permit maximum cooling capacity of the refrigeration system upon start-up, particularly when the space being cooled is particularly warm, such as the inside of an automobile that has been in the sun. When a reduced ambient temperature is attained, or when a reduced suction line temperature is attained, the expansion valve is automatically re-set to its normal operating parameters.
U.S. Pat. No. 3,791,165 also relates to a charging method and apparatus for a refrigeration system and is specifically applicable to a refrigeration system having a fixed restriction refrigerant expansion valve. Proper operation of such a refrigeration system is achieved by adding or removing refrigerant to the system to attain a preselected super-heat temperature of the refrigerant leaving the evaporator coil as determined by comparing the pressure and temperature of such refrigerant gas.
U.S. Pat. No. 3,803,863 relates to a system for controlling a refrigeration compressor which involves monitoring the super-heat contained in the refrigerant gas returning to the compressor, monitoring the temperature of the space to be cooled as compared with a set point, generating separate electrical signals correlated to the super-heat temperature and the differential between the set point and the space temperature, and utilizing these signals to produce a modulating signal for regulating the compressor operation in the refrigeration system.
U.S. Pat. No. 3,803,864 also relates to an air conditioning control system which utilizes a normally constant pressure expansion device for admitting liquid refrigerant to the evaporator but which is adapted to adjust the expansion device to maintain a relatively high evaporator pressure during the time that the temperature in the space to be cooled is being reduced to its desired level. When the space temperature has reached the desired level, the expansion device then reverts to its normal operation.
U.S. Pat. No. 3,803,865 utilizes two vacuum control valves, one in the feed line between the condensor and the evaporator and another in the suction line between the evaporator and the compressor. The vacuum port of the first mentioned valve is connected to the suction line while the suction port of the second valve is connected through appropriate conduit to the induction system of an automotive engine. Application of suction to the second valve results in a pneumatic signal being transmitted to the first valve to increase the control point at which the evaporator pressure is controlled.
Lastly, U.S. Pat. No. 3,913,347 relates to a mechanically operated switching arrangement controlled by pressure of refrigerant in the suction line on the one hand, and by pressure as it is related to the temperature of the refrigerant gas in the suction line on the other hand. Pressure responsive bellows are opposed to each other and each is in contact with a lever pivoted in such a manner to open or close or neutralize a pair of contacts, depending upon the differential in pressure as exerted directly by the pressure of the suction line and the pressure exerted by heating an appropriate refrigerant by means of the heat contained in the refrigerant gas.
From the above prior art it will be apparent that there have been many different mechanical devices utilized that respond directly to variations of pressure of the refrigerant gas in the suction line, and which respond to variations in pressure in an auxiliary bulb containing an appropriate refrigerant gas and responsive to temperature variations of the refrigerant gas in the suction line. Most of these devices, as indicated in the patents discussed above, are mechanical devices with the disadvantages inherent in such mechanical device, such as slow response time, different characteristics because of inability to maintain manufacturing tolerances, and space limitations that preclude many of these cumbersome mechanical devices to be retro-fitted to existing equipment. Accordingly, it is one of the principal objects of the present invention to provide a control device for air conditioning refrigeration systems that is almost instantaneous in its response time and which may be easily retro-fitted to existing air conditioning refrigeration systems.
In the operation of an air conditioning refrigeration system it frequently happens that the super-heat in the suction line will fluctuate through a relatively wide range in a very short period of time. Such fluctuations occur in most refrigeration systems and are usually not harmful because their duration is a relatively short period of time. With the mechanical structures disclosed in the patents above, such fluctuations would have their normal and expected effect on the mechanical transducers connected to the sensors and, because of the inherent lag time in the mechanical devices, the system might be shut down despite the fact that the super-heat fluctuation no longer exists and the super-heat is approaching a normal value. Accordingly, another object of the invention is to provide a control device for air conditioning refrigeration systems which is responsive to such extreme fluctuations of super-heat within a prescribed range and which is effective to delay the effect of such fluctuations so as to preclude shutting down the refrigeration system unnecessarily.
It is particularly surprising that it is not revealed in any of the patents disclosed above or in operation manuals and texts on refrigeration that there is a substantially linear relationship between the suction line pressure and the temperature of the refrigerant gas flowing through the suction line. I have found that when this relationship is defined as a ratio of change of the pressure in the suction line to a change of temperature of the refrigerant gas passing therethrough, as long as the ratio is maintained, the super-heat in the refrigerant gas returning to the compressor may vary over a relatively wide range without the need to activate protective devices. Accordingly, a still further object of this invention is to provide a control device in which a predetermined super-heat temperature may be selected as the optimum super-heat for a given system, and the device set to initiate remedial steps only if the super-heat either drops a significant predetermined amount below such set super-heat temperature or rises a significant predetermined amount above the preset super-heat temperature, and to not initiate remedial steps so long as the ratio of change of pressure and temperature remains constant within the preselected range.
It frequently happens that an abnormality in the operation of a refrigeration system will cause either a loss or a gain of the super-heat in the refrigerant gas returning to the compressor. Frequently the malfunction that causes the loss or gain in super-heat cures itself within a short interval. It is a disadvantage to have the system shut down because of a temporary abnormality in the operation of the system. Accordingly, still another object of the present invention is to provide a control device that initiates a counter which locks out other protective devices for a predetermined interval to thus permit the system a sufficient time to return to its normal mode of operation without shutting down the system.
The invention possesses other objects and features of value, some of which, with the foregoing, will be apparent from the following description and the drawings. It is to be understood however that the invention is not limited to the embodiment illustrated and described since it may be embodied in various forms within the scope of the appended claims.